[docs] remove pymdownx.githubemoji from config

rogerwang · rogerwang · commit ddf202351007 · 2017-04-20T10:33:44.000+08:00
The build is failing with: https://readthedocs.org/projects/nwjs/builds/5312824/
diff --git a/mkdocs.yml b/mkdocs.yml
@@ -14,7 +14,6 @@ markdown_extensions:
     - pymdownx.tilde
     - pymdownx.caret
     - pymdownx.smartsymbols
-    - pymdownx.githubemoji
     - pymdownx.tasklist
     - pymdownx.progressbar
     - pymdownx.headeranchor
